Electric resistance welded steel pipe for torsion beam

ABSTRACT

An electric resistance welded steel pipe for a torsion beam, in which a chemical composition of a base metal portion contains, in terms of % by mass, 0.05 to 0.30% of C, 0.03 to 1.20% of Si, 0.30 to 2.50% of Mn, 0.010 to 0.200% of Ti, 0.005 to 0.500% of Al, 0.010 to 0.040% of Nb, and 0.0005 to 0.005(W % of B, the balance containing Fe and impurities, wherein: Vc90, defined by the following Formula (i), is from 2 to 150, a mass ratio Ti/N is 3.4 or more, a microstructure of a wall thickness central portion in an L cross section at a base metal 1800 position is a tempered martensite, an average aspect ratio of prior γ grains in the tempered martensite is 2.0 or less, and a tensile strength in the pipe axis direction is from 750 to 980 MPa:log Vc90=2.94−0.75βa  Formula (i)βa=2.7C+0.4Si+Mn+0.45Ni+0.8Cr+2Mo  Formula (ii).

TECHNICAL FIELD

The present disclosure relates to an electric resistance welded steelpipe for a torsion beam.

BACKGROUND ART

Conventionally, studies have been made on steel materials used forautomobile structural members (for example, automotive underbody parts).

For example, Patent Document 1 discloses a hot-rolled steel sheet for amechanical structure steel pipe excellent in fatigue characteristics andbending formability, which is used for a mechanical structure steel pipesuch as an automotive underbody part steel pipe.

Patent Document 1: International Publication No. WO 2009/133965

SUMMARY OF INVENTION Technical Problem

High tensile strength (in particular, tensile strength in the pipe axisdirection) is required for a torsion beam, which is one of automotiveunderbody parts.

Meanwhile, a torsion beam may be produced by processing an electricresistance welded steel pipe for torsion beam by bending forming. Insuch a case, cracks (hereinafter also referred to as “inner surfacecracks”) may be generated on the inner surface of the bent-formedportion (hereinafter also referred to as “bent portion”) of the electricresistance welded steel pipe. From the viewpoint of bending formabilityof an electric resistance welded steel pipe, there are cases in which itis required to improve the inner surface crack resistance of an electricresistance welded steel pipe.

The term “inner surface cracking resistance” used herein means aproperty capable of suppressing inner surface cracking when processingan electric resistance welded steel pipe by bending forming.

In Patent Document 1, no examination has been made from the viewpoint ofimproving the inner surface cracking resistance of the steel pipe,leaving room for further improvement.

An object of the disclosure is to provide an electric resistance weldedsteel pipe for a torsion beam having excellent tensile strength in apipe axis direction and also excellent inner surface crackingresistance.

Solution to Problem

Means for solving the problem described above includes the followingaspects.

<1> An electric resistance welded steel pipe for a torsion beam, thesteel pipe comprising a base metal portion and an electric resistancewelded portion,

wherein a chemical composition of the base metal portion consists of, interms of % by mass:

0.05 to 0.30% of C,

0.03 to 1.20% of Si,

0.30 to 2.50% of Mn,

0 to 0.030% of P,

0 to 0.010% of S,

0.010 to 0.200% of Ti,

0.005 to 0.500% of Al,

0.010 to 0.040% of Nb.

0 to 0.006% of N,

0.0005 to 0.0050% of B.

0 to 1.000% of Cu,

0 to 1.000% of Ni.

0 to 1.00% of Cr,

0 to 0.50% of Mo,

0 to 0.200% of V,

0 to 0.100% of W.

0 to 0.0200% of Ca.

0 to 0.0200% of Mg,

0 to 0.0200% of Zr,

0 to 0.0200% of REM, and,

a balance consisting of Fe and impurities, wherein:

V_(c90), defined by the following Formula (i), is from 2 to 150,

a mass ratio of Ti content to N content is 3.4 or more,

a metallographic microstructure of a wall thickness central portion is atempered martensite structure, and an average aspect ratio of prioraustenite grains in the tempered martensite structure is 2.0 or less, inan L cross section at a position deviating by 180° in a circumferentialdirection of the pipe from the electric resistance welded portion.

a metallographic microstructure of an area within a distancecorresponding to a wall thickness from the electric resistance weldedportion in a wall thickness central portion in a C cross sectionincludes a tempered martensite and at least one of a tempered bainite ora ferrite,

yield elongation is observed when a tensile test in a pipe axisdirection is performed, and

a tensile strength in the pipe axis direction is from 750 to 980 MPa:

log V_(c90)=2.94−0.75βa  Formula (i)

βa=2.7C+0.4Si+Mn+0.45Ni+0.8Cr+2Mo  Formula (ii)

wherein, in Formula (i), Pa represents a value defined by Formula (ii),and in Formula (ii), element symbols represent % by mass of respectiveelements.

<2> The electric resistance welded steel pipe for a torsion beamaccording to <1>, wherein the chemical composition of the base metalportion contains, in terms of % by mass, at least one selected from thegroup consisting of.

more than 0% but equal to or less than 1.000% of Cu,

more than 0% but equal to or less than 1.000% of Ni,

more than 0% but equal to or less than 1.00% of Cr,

more than 0% but equal to or less than 0.50% of Mo,

more than 0% but equal to or less than 0.200% of V,

more than 0% but equal to or less than 0.100% of W,

more than 0% but equal to or less than 0.0200% of Ca.

more than 0% but equal to or less than 0.0200% of Mg,

more than 0% but equal to or less than 0.0200% of Zr, and

more than 0% but equal to or less than 0.0200% of REM.

<3> The electric resistance welded steel pipe for a torsion beamaccording to <1> or <2>, wherein packet grains in the temperedmartensite structure have an average grain size of 10 μm or less.<4> The electric resistance welded steel pipe for a torsion beamaccording to any one of <1> to <3>, wherein the wall thickness centralportion in the L cross section has a dislocation density of 2.0×10¹⁴ m⁻²or less.<5> The electric resistance welded steel pipe for a torsion beamaccording to any one of <1> to <4>, which has an outer diameter of from50 to 150 mm and a wall thickness of from 2.0 to 4.0 mm.

Advantageous Effects of Invention

According to the disclosure, an electric resistance welded steel pipefor a torsion beam having excellent tensile strength in a pipe axisdirection and also excellent inner surface cracking resistance isprovided.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross-sectional view conceptually showing a partof the C cross section of the electric resistance welded steel pipeaccording to an example of the disclosure, which is a figure forexplaining an area within a distance corresponding to a wall thicknessfrom the electric resistance welded portion (i.e., a vicinity of theelectric resistance welded portion) in a wall thickness central portionin a C cross section.

FIG. 2 is a schematic view showing the outline of the bending test inthe Examples.

FIG. 3 is a schematic cross-sectional view schematically showing a crosssection of a structure obtained by processing an electric resistancewelded steel pipe by bending forming in the bending test in theExamples.

DESCRIPTION OF EMBODIMENTS

A numerical range expressed by “x to y” herein includes the values of xand y in the range as the minimum and maximum values, respectively.

The term “step” herein encompasses not only an independent step but alsoa step of which the desired object is achieved even in a case in whichthe step is incapable of being definitely distinguished from anotherstep.

In the numerical ranges described herein as stepwise ranges, the upperlimit value or the lower limit value of a certain stepwise numericalrange may be replaced by the upper limit value or the lower limit valueof a different stepwise numerical range, and may also be replaced by avalue set forth in Examples.

The content of a component (element) expressed by “%” herein means “% bymass”.

The content of C (carbon) may be herein occasionally expressed as “Ccontent”. The content of another element may be expressed similarly.

Herein, the “L cross section” refers to a cross section parallel to apipe axis direction and a wall thickness direction, and the “C crosssection” refers to a cross section perpendicular to a pipe axisdirection.

The electric resistance welded steel pipe for a torsion beam(hereinafter also simply referred to as “electric resistance weldedsteel pipe”) of the disclosure includes a base metal portion and anelectric resistance welded portion, wherein a chemical composition ofthe base metal portion consists of, in terms of % by mass: 0.05 to 0.30%of C, 0.03 to 1.20% of Si, 0.30 to 2.50% of Mn, 0 to 0.030% of P, 0 to0.010% of S, 0.010 to 0.200% of Ti, 0.005 to 0.500% of Al, 0.010 to0.040% of Nb, 0 to 0.006% of N, 0.0005 to 0.0050% of B, 0 to 1.000% ofCu, 0 to 1.000% of Ni, 0 to 1.00% of Cr, 0 to 0.50% of Mo, 0 to 0.200%of V, 0 to 0.100% of W, 0 to 0.0200% of Ca, 0 to 0.0200% of Mg, 0 to0.0200% of Zr, REM: 0 to 0.0200%, and a balance consisting of Fe andimpurities, wherein: V_(c90), defined by the following Formula (i), isfrom 2 to 150, a mass ratio of Ti content to N content is 3.4 or more, ametallographic microstructure of a wall thickness central portion is atempered martensite structure, and an average aspect ratio of prioraustenite grains in the tempered martensite structure is 2.0 or less, inan L cross section at a position deviating by 180° in a circumferentialdirection of the pipe from the electric resistance welded portion, ametallographic microstructure of an area within a distance correspondingto a wall thickness from the electric resistance welded portion in awall thickness central portion in a C cross section includes a temperedmartensite and at least one of a tempered bainite or a ferrite, yieldelongation is observed when a tensile test in a pipe axis direction isperformed, and a tensile strength in the pipe axis direction is from 750to 980 MPa.

log V_(c90)=2.94−0.75βa  Formula (i)

βa=2.7C+0.4Si+Mn+0.45Ni+0.8Cr+2Mo  Formula (ii)

In Formula (i), Pa represents a value defined by Formula (ii), and inFormula (ii), element symbols represent % by mass of respectiveelements.

Herein, the chemical composition of the base metal portion describedabove (including that V_(c90) is from 2 to 150 and that the mass ratioof Ti content to N content is 3.4 or more) is also referred to as“chemical composition in the disclosure.”

The electric resistance welded steel pipe of the disclosure comprises abase metal portion and an electric resistance welded portion.

An electric resistance welded steel pipe is generally produced byforming a hot-rolled steel sheet into a tubular shape (hereinafter alsoreferred to as “roll-forming”) to thereby make an open pipe, andprocessing abutting portions of the obtained open pipe by electricresistance welding to form an electric resistance welded portion(hereinafter, the process up to this point is also referred to as“pipe-making”), and then, if necessary, performing seam heat treatmentof the electric resistance welded portion.

The electric resistance welded steel pipe of the disclosure is producedby performing tempering after pipe-making (seam heat treatment in a casein which seam heat treatment is performed) (hereinafter also referred toas “tempering after pipe-making”).

In the electric resistance welded steel pipe of the disclosure, the basemetal portion refers to a portion other than the electric resistancewelded portion and a heat affected zone.

The heat affected zone (hereinafter also referred to as “HAZ”) refers toa portion affected by heat caused by electric resistance welding(affected by heat caused by the electric resistance welding and seamheat treatment in a case in which the seam heat treatment is performedafter the electric resistance welding).

The heat affected zone described herein and an area within a distancecorresponding to a wall thickness from an electric resistance weldedportion described later (hereinafter also referred to as a “vicinity ofan electric resistance welded portion”) have an overlapping portion.

The hot-rolled steel sheet, which is a material of an electricresistance welded steel pipe, is manufactured by using a hot strip mill.Specifically, a long hot-rolled steel sheet coiled into a coil(hereinafter, also referred to as a “hot coil”) is produced by a hotstrip mill.

A hot-rolled steel sheet, which is a material of an electric resistancewelded steel pipe, is different from a steel plate produced by using aplate mill in that it is a continuous steel sheet.

Since a steel plate is not a continuous steel sheet, it cannot be usedfor roll-forming, which is a continuous bending process.

An electric resistance welded steel pipe is clearly distinguished from awelded steel pipe (e.g., UOE steel pipe) produced by using a steel platein that it is produced by using the hot-rolled steel sheet describedabove.

The electric resistance welded steel pipe of the disclosure hasexcellent tensile strength in a pipe axis direction (specifically, atensile strength in a pipe axis direction of 750 MPa or more) and alsoexcellent inner surface cracking resistance.

The reason for obtaining these effects are assumed as follows. However,the electric resistance welded steel pipe of the disclosure is notlimited to the following assumed reasons.

The effect of excellent tensile strength in the pipe axis direction isattributed to that the base metal portion has the chemical compositionin the disclosure, and that the metallographic microstructure of thewall thickness central portion in the L cross section is the temperedmartensite structure.

The effect of excellent inner surface cracking resistance is attributedto that the metallographic microstructure of the wall thickness centralportion in the L cross section at the position deviating by 180° in thecircumferential direction of the pipe from the electric resistancewelded portion (hereinafter also referred to as “base metal 180°position”) is a tempered martensite structure.

The base metal 180° position refers herein to a position selected as arepresentative position of the base metal portion.

In contrast to the electric resistance welded steel pipe of thedisclosure, in a case in which the metallographic microstructure of awall thickness central portion in the L cross section at the base metal180° position has a dual-phase structure consisting of, for example, atempered martensite and another structure (e.g., a ferrite, a temperedbainite, or the like), the inner surface cracking resistancedeteriorates. This is thought to be because during bending forming of anelectric resistance welded steel pipe, forming strain due to bendingforming is concentrated on the boundary between two structures havingdifferent hardness on the inner surface of the bent portion and itsvicinity, and as a result, inner surface cracking is likely to occur.

In addition, it is thought that the effect of excellent inner surfacecracking resistance is also attributed to that the average aspect ratioof prior austenite grains in the tempered martensite structure is 2.0 orless.

In contrast to the electric resistance welded steel pipe of thedisclosure, in a case in which the average aspect ratio of prioraustenite grains in the tempered martensite structure in themetallographic microstructure of a wall thickness central portion in theL cross section exceeds 2.0, the inner surface cracking resistancedeteriorates.

This is thought to be because in a case in which the average aspectratio of prior austenite grains exceeds 2.0 (i.e., in a case in whichprior austenite is elongated), the aspect ratio of packet grains inprior austenite grains also increases (i.e., packet grains are alsoelongated), and as a result, the packet grain boundary is easilycontinuous. It is therefore considered that during bending forming, thetearing of the packet grain boundary tends to extend along the packetgrain boundary in the inner surface of the bent portion and itsvicinity, and as a result, the inner surface cracking which iscontinuous cracking tends to occur.

In the electric resistance welded steel pipe of the disclosure, theaverage aspect ratio of prior austenite grains is set to 2.0 or less,and thereby, the shape of the packet grains in prior austenite grains ismade closer to a spherical shape. As a result, it is considered that thecontinuity of the packet grain boundary is suppressed, the extension ofthe tearing of the packet grain boundary described above is suppressed,and as a result, the inner surface cracking is suppressed (i.e., theinner surface cracking resistance is improved).

The tempered martensite structure of the wall thickness central portionin the L cross section at the base metal 180° position is structuredwith a combination of the chemical composition in the disclosure and theelectric resistance welded steel pipe production conditions (includingconditions of producing a hot-rolled steel sheet which is a material ofan electric resistance welded steel pipe).

Specifically, in the hot rolling process and cooling process forproducing a hot-rolled steel sheet having the chemical composition inthe disclosure, substantial quenching is performed, and an as-quenchedmartensite structure (i.e., an untempered martensite structure; the sameapplies hereinafter) is structured as the structure of the base metalportion (typically, at the base metal 180° position), and then, atempered martensite structure is created by tempering after pipe-making.

In addition, it is possible to achieve an average aspect ratio of prioraustenite grains in the tempered martensite structure of 2.0 or less asdescribed above by performing rolling in a recrystallization region inthe hot rolling process (e.g., by setting the hot rolling finishingtemperature to 920° C. or higher).

An example of the method of producing the electric resistance weldedsteel pipe of the disclosure will be described later.

In the electric resistance welded steel pipe of the disclosure, ametallographic microstructure of a vicinity of an electric resistancewelded portion (i.e., an area within a distance corresponding to a wallthickness from the electric resistance welded portion) in a wallthickness central portion in a C cross section includes a temperedmartensite and at least one of a tempered bainite or a ferrite.

The metallographic microstructure of the vicinity of the electricresistance welded portion includes at least one of a tempered bainite ora ferrite, which indicates that the electric resistance welded steelpipe of the disclosure is an electric resistance welded steel pipe thathas been tempered without quenching after pipe making.

In contrast to the electric resistance welded steel pipe of thedisclosure, in the case of an electric resistance welded steel pipe thathas been quenched and tempered after-pipe making, a vicinity of anelectric resistance welded portion has a tempered martensite structurewhich is substantially free of a tempered bainite and a ferrite.

A production method for producing the electric resistance welded steelpipe of the disclosure by performing tempering without quenching afterpipe-making is superior in productivity as compared with a productionmethod in which quenching and tempering are performed after pipe-making.

In other words, the electric resistance welded steel pipe of thedisclosure is also advantageous in that it is excellent in economy(i.e., low cost) because it can be produced by a production method whichis more productive than a production method in which quenching andtempering are performed after pipe-making.

In addition, in the electric resistance welded steel pipe of thedisclosure, yield elongation is observed when a tensile test in the pipeaxis direction is performed, which also indicates that the electricresistance welded steel pipe of the disclosure is an electric resistancewelded steel pipe that has been tempered without quenching afterpipe-making.

In contrast to the electric resistance welded steel pipe of thedisclosure, yield elongation is not observed in a case in which atensile test in the pipe axis direction is performed on an electricresistance welded steel pipe that has been tempered before pipe-making(i.e., a hot-rolled steel sheet as a material of the pipe has beentempered) but has not been tempered after pipe-making (see, for example,Comparative Example 25 described later).

The electric resistance welded steel pipe of the disclosure is superiorin inner surface cracking resistance as compared with an electricresistance welded steel pipe that has been tempered before pipe-makingbut has not been tempered after pipe-making. This is thought to bebecause, in the electric resistance welded steel pipe of the disclosure,the pipe-making strain generated during pipe-making is reduced bytempering after pipe-making, thereby reducing the dislocation density.

<Chemical Composition of Base Metal Portion>

The chemical composition of the base metal portion of the electricresistance welded steel pipe of the disclosure (i.e., “chemicalcomposition in the disclosure”) is described below.

C: 0.05 to 0.30%

C is an element for improving the strength of steel. In a case in whichthe C content is less than 0.05%, the strength required for a torsionbeam might not be achieved. Accordingly, the C content is 0.05% or more.The C content is preferably 0.08% or more, and more preferably 0.10% ormore.

Meanwhile, in a case in which the C content exceeds 0.30%, the strengthmight be excessively increased, resulting in deterioration of innersurface cracking resistance. Accordingly, the C content is 0.30% orless. The C content is preferably 0.25% or less, and more preferably0.20% or less.

Si: 0.03 to 1.20%

Si is an element used for deoxidation. In a case in which the Si contentis less than 0.03%, deoxidation might become insufficient, resulting ingeneration of a coarse Fe-oxide. Accordingly, the Si content is 0.03% ormore. The Si content is preferably 0.10% or more, and more preferably0.20% or more.

Meanwhile, in a case in which the Si content exceeds 1.20%, it mightcause generation of an inclusion such as SiO₂, thereby facilitatinggeneration of microvoids starting from the inclusion during roll-formingfor producing an electric resistance welded steel pipe and/or bendingforming on an electric resistance welded steel pipe. Accordingly, the Sicontent is 1.20% or less. The Si content is preferably 1.00% or less,and more preferably 0.80% or less.

Mn: 0.30 to 2.50%

Mn is an important element for enhancing hardenability to improve thestrength of steel, and eventually (i.e., by tempering afterpipe-forming) to obtain a tempered martensite structure. If the Mncontent is less than 0.30%, hardenability may be insufficient, and atempered martensite structure may not be obtained. Accordingly, the Mncontent is 0.30% or more. The Mn content is preferably 0.60% or more,and more preferably 0.70% or more.

Meanwhile, in a case in which the Mn content exceeds 2.50%, the strengthmight be excessively increased, resulting in deterioration of innersurface cracking resistance. Accordingly, the Mn content is 2.50% orless. The Mn content is preferably 2.00% or less, more preferably 1.50%or less, and still more preferably 1.30% or less.

P: 0 to 0.030%

P is an element that can be contained as an impurity in steel. In a casein which the P content exceeds 0.030%, it might facilitate concentrationof P in the packet grain boundary, resulting in deterioration of innersurface cracking resistance. Accordingly, the P content is 0.030% orless. The P content is preferably 0.020% or less.

Meanwhile, the P content may be 0%. From the viewpoint of reducing adephosphorization cost, the P content may be more than 0%, 0.001% ormore, or 0.010% or more.

S: 0 to 0.010%

S is an element that can be contained as an impurity in steel.

In a case in which the S content exceeds 0.010%, it might causegeneration of coarse MnS, resulting in deterioration of inner surfacecracking resistance. Accordingly, the S content is 0.010% or less. The Scontent is preferably 0.005% or less.

Meanwhile, the S content may be 0%. From the viewpoint of reducing adesulfunzation cost, the S content may be more than 0%, 0.001% or more,or 0.003% or more.

Ti: 0.010 to 0.200%

Ti is an element that improves the strength of steel by precipitating asTiC. Ti is also an element that contributes to austenite grain sizerefining through the pinning effect in hot rolling, and as a result,contributes to refining of packet grains in prior austenite grains. In acase in which the Ti content is less than 0.010%, the strength requiredfor a torsion beam and the pinning effect on austenite grains might notbe achieved. In addition, in a case in which the Ti content is less than0.010%, since N cannot be fixed as TiN and N precipitates together withB (i.e., BN is formed), an effective amount of B that contributes to theimprovement of hardenability cannot be secured, and as a result, theeffect of improving hardenability by B may not be obtained. Accordingly,the Ti content is 0.010% or more. The Ti content is preferably 0.015% ormore.

Meanwhile, in a case in which the Ti content exceeds 0.200%, it mightcause precipitation of coarse TiC and/or TiN, resulting in deteriorationof inner surface cracking resistance. Accordingly, the Ti content is0.200% or less. The Ti content is preferably 0.150% or less, morepreferably 0.120% or less, still more preferably 0.100% or less, andfurthermore preferably 0.080% or less.

Al: 0.005 to 0.500%

Al is an element that forms AlN, contributes to austenite grain sizerefining through the pinning effect in hot rolling, and as a result,contributes to refining of packet grains in prior austenite grains. In acase in which the Al content is less than 0.005%, the pinning effect onaustenite grains cannot be obtained, which results in coarse prioraustenite grains, and as a result, packet grains may become coarse.Accordingly, the Al content is 0.005% or more. The Al content ispreferably 0.010% or more, and more preferably 0.020% or more.

Meanwhile, in a case in which the Al content exceeds 0.500%, it mightcause precipitation of coarse AlN resulting in deterioration of innersurface cracking resistance. Accordingly, the Al content is 0.500% orless. The Al content is preferably 0.100% or less, and more preferably0.050% or less.

Nb: 0.010 to 0.040%

Nb is an element that forms NbC, contributes to austenite grain sizerefining through the pinning effect in hot rolling, and as a result,contributes to refining of packet grains in prior austenite grains. In acase in which the Nb content is less than 0.010%, the pinning effect onaustenite grains cannot be obtained, which results in coarse prioraustenite grains, and as a result, packet grains may become coarse.Accordingly, the Nb content is 0.010% or more. The Nb content ispreferably 0.020% or more.

Meanwhile, in a case in which the Nb content exceeds 0.040%, it mightcause precipitation of coarse NbC resulting in deterioration of innersurface cracking resistance. Accordingly, the Nb content is 0.040% orless. The Nb content is preferably 0.036% or less.

N: 0 to 0.006%

N is an element that can be contained as an impurity in steel. In a casein which the N content exceeds 0.006%, it might cause generation ofcoarse AlN, resulting in deterioration of inner surface crackingresistance. Accordingly, the N content is 0.006% or less.

The N content may also be 0%.

N is an element that forms AlN, contributes to austenite grain sizerefining through the pinning effect in hot rolling, and as a result, itis also an element that contributes to refining of packet grains inprior austenite grains. From the viewpoint of such an effect, the Ncontent may be more than 0%, or 0.001% or more.

B: 0.0005 to 0.0050%

B is an element that improves the hardenability of steel, and is anessential element for creating an as-quenched martensite structure inthe hot rolling process and cooling process for producing a hot-rolledsteel sheet that is a material for an electric resistance welded steelpipe. In a case in which the B content is less than 0.0005%, the effectmay not be obtained. Accordingly, the B content is 0.0005% or more. TheB content is preferably 0.0010% or more.

Meanwhile, in a case in which the B content exceeds 0.0050%, since Baggregates and/or precipitates and the solid solution B segregated atthe austenite grain boundary decreases, the effect of improving thehardenability of steel may decrease. Accordingly, the B content is,0.0050% or less. The B content is preferably 0.0040% or less, and morepreferably 0.0030% or less.

Cu: 0 to 1.000%

Cu is an optional element, and thus it may not be contained. In otherwords, the Cu content may be 0%.

Cu is an element that contributes to enhancement of the strength ofsteel. From the viewpoint of such an effect, the Cu content may be morethan 0%, 0.005% or more, 0.010% or more, or 0.050% or more.

Meanwhile, in a case in which the Cu content is excessively increased,it might cause saturation of the effect, leading to cost increase.Accordingly, the Cu content is 1.000% or less. The Cu content ispreferably 0.500% or less, more preferably 0.300% or less, and stillmore preferably 0.200% or less.

Ni: 0 to 1.000%

Ni is an optional element, and thus it may not be contained. In otherwords, the Ni content may be 0%.

Ni is an element that contributes to enhancement of the strength ofsteel. From the viewpoint of such an effect, the Ni content may be morethan 0%, 0.005% or more, 0.010% or more, or 0.050% or more.

Meanwhile, in a case in which the Ni content is excessively increased,it might cause saturation of the effect, leading to cost increase.Accordingly, the Ni content is 1.000% or less. The Ni content ispreferably 0.500% or less, more preferably 0.300% or less, and stillmore preferably 0.200% or less.

Cr: 0 to 1.00%

Cr is an optional element, and thus it may not be contained. In otherwords, the Cr content may be 0%.

Cr is an element that contributes to enhancement of the strength ofsteel. From the viewpoint of such an effect, the Cr content may be morethan 0%, 0.005% or more, 0.01% or more, or 0.05% or more.

Meanwhile, in a case in which the Cr content is excessively increased,it might cause saturation of the effect, leading to cost increase.Accordingly, the Cr content is 1.00% or less. The Cr content ispreferably 0.50% or less, more preferably 0.30% or less, and still morepreferably 0.20%.

Mo: 0 to 0.50%

Mo is an optional element, and thus it may not be contained. In otherwords, the Mo content may be 0%.

Mo is an element that contributes to enhancement of the strength ofsteel. From the viewpoint of such an effect, the Mo content may be morethan 0%, 0.01% or more, 0.05% or more, or 0.10% or more.

Meanwhile, in a case in which the Mo content is excessively increased,it might cause saturation of the effect, leading to cost increase.Accordingly, the Mo content is 0.50% or less. The Mo content ispreferably 0.40% or less.

V: 0 to 0.200%

V is an optional element, and thus it may not be contained. In otherwords, the V content may be 0%.

V is an element that contributes to enhancement of the strength ofsteel. From the viewpoint of such an effect, the V content may be morethan 0%, or 0.005% or more.

Meanwhile, in a case in which the V content is excessively increased, itmight cause saturation of the effect, leading to cost increase.Accordingly, the V content is 0.200% or less. The V content ispreferably 0.100% or less, and more preferably 0.050% or less.

W: 0 to 0.100%

W is an optional element, and thus it may not be contained. In otherwords, the W content may be 0%.

W is an element that contributes to enhancement of the strength ofsteel. From the viewpoint of such an effect, the W content may be morethan 0%, or 0.005% or more.

Meanwhile, in a case in which the W content is excessively increased, itmight cause saturation of the effect, leading to cost increase.Accordingly, the W content is 0.100% or less. The W content ispreferably 0.050% or less.

Ca: 0 to 0.0200%

Ca is an optional element, and thus it may not be contained. In otherwords, the Ca content may be 0%.

Ca has effects of controlling an inclusion and further suppressing innersurface cracking resistance. From the viewpoint of such an effect, theCa content may be more than 0%, 0.0001% or more, or 0.0010% or more.

Meanwhile, in a case in which the Ca content is excessively increased,it might cause generation of coarse Ca-oxide, resulting in deteriorationof inner surface cracking resistance. Accordingly, the Ca content is0.0200% or less. The Ca content is preferably 0.0100% or less, and morepreferably 0.0070% or less.

Mg: 0 to 0.0200%

Mg is an optional element, and thus it may not be contained. In otherwords, the Mg content may be 0%.

Mg has effects of controlling an inclusion and further suppressing innersurface cracking resistance. From the viewpoint of such an effect, theMg content may be more than 0%, or 0.0001% or more.

Meanwhile, in a case in which the Mg content is excessively increased,it might cause saturation of the effect, leading to cost increase.Accordingly, the Mg content is 0.0200% or less. The Mg content ispreferably 0.0100% or less, more preferably 0.0050% or less, and stillmore preferably 0.0020% or less.

Zr: 0 to 0.0200%

Zr is an optional element, and thus it may not be contained. In otherwords, the Zr content may be 0%.

Zr has effects of controlling an inclusion and further suppressing innersurface cracking resistance. From the viewpoint of such an effect, theZr content may be more than 0%, or 0.0001% or more.

Meanwhile, in a case in which the Zr content is excessively increased,it might cause saturation of the effect, leading to cost increase.Accordingly, the Zr content is 0.0200% or less. The Zr content ispreferably 0.0100% or less, more preferably 0.0050% or less, and stillmore preferably 0.0020% or less.

REM: 0 to 0.0200%

REM is an optional element, and thus it may not be contained. In otherwords, the REM content may be 0%.

“REM” refers to a rare earth element, i.e., at least one elementselected from the group consisting of Sc, Y, La. Ce, Pr, Nd, Pm, Sm, Eu,Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu.

REM has effects of controlling an inclusion and further suppressinginner surface cracking resistance. From the viewpoint of such an effect,the REM content may be more than 0%, 0.0001% or more, or 0.0005% ormore.

Meanwhile, in a case in which the REM content is excessively increased,it might cause generation of coarse oxide, resulting in deterioration ofinner surface cracking resistance. Accordingly, the REM content is0.0200% or less. The REM content is preferably 0.0100% or less, morepreferably 0.0050% or less, and still more preferably 0.0020% or less.

Balance: Fe and Impurities

In the chemical composition of the base metal portion, the balanceexcluding each element described above is Fe and impurities.

The impurities refer to components which are contained in a raw material(for example, ore, scrap, or the like) or mixed into in a productionstep, and which are not intentionally incorporated into a steel.

Examples of the impurities include any elements other than the elementsdescribed above. Elements as the impurities may be only one kind, or maybe two or more kinds.

Examples of the impurities include Sb, Sn, Co. As, Pb, Bi. and H.

Typically, Sb, Sn. Co, or As may be included in a content of, forexample, 0.1% or less, Pb or Bi may be included in a content of, forexample, 0.005% or less, H may be included in a content of, for example,0.0004% or less, and the contents of the other elements need notparticularly be controlled as long as being in a usual range.

From the viewpoint of obtaining the above effects of each element, thechemical composition of a base metal portion may include one or morekinds selected from the group consisting of: more than 0% but equal toor less than 1.000% of Cu, more than 0% but equal to or less than 1.000%of Ni, more than 0% but equal to or less than 1.00% of Cr, more than 0%but equal to or less than 0.50% of Mo, more than 0% but equal to or lessthan 0.200% of V, more than 0% but equal to or less than 0.100% of W,more than 0% but equal to or less than 0.0200% of Ca, more than 0% butequal to or less than 0.0200% of Mg, more than 0% but equal to or lessthan 0.0200% of Zr, and more than 0% but equal to or less than 0.0200%of REM.

The preferred ranges of the contents of these elements are as describedabove.

V_(c90): from 2 to 150

In the chemical composition of the base metal portion. V_(c90), definedby the following Formula (i), represents a value as an index of thehardenability of steel.

V_(c90) is a value known as the critical cooling rate (unit: ° C./s) atwhich a 90% martensite structure is obtained (e.g., see Ueno et al.'spaper “New Empirical Formula for Estimating the Hardenability in placeof GROSSMANN'S Equation.” “Tetsu-to-Hagane” (The Iron and SteelInstitute of Japan), Vol. 74 (1988). No. 6, pp. 1073-1080).

log V_(c90)=2.94−0.75βa  Formula (i)

βa=2.7C+0.4Si+Mn+0.45Ni+0.8Cr+2Mo  Formula (ii)

[In Formula (i), Pa represents a value defined by Formula (ii), and inFormula (ii), element symbols represent % by mass of respectiveelements.]

As V_(c90) decreases, the hardenability of steel increases.

Therefore, in a case in which V_(c90) is 150 or less, since theformation of a ferrite and a bainite is suppressed and the formation ofan as-quenched martensite is promoted, it is easy to obtain a temperedmartensite structure by tempering after pipe-making.

In addition, in a case in which V_(c90) is 2 or more, it is advantageousin terms of cost.

In order to reduce V_(c90) to less than 2, it is necessary to add alarge amount of alloy elements, which requires a lot of time and costwhen refining in the steelmaking process.

Therefore, V_(c90) is 2 to 150.

The upper limit of V_(c90) is preferably 140.

The lower limit of V_(c90) is preferably 10, and more preferably 20.

Mass ratio of Ti content to N content: 3.4 or more

In the chemical composition of the base metal portion, the mass ratio ofTi content to N content (hereinafter, also referred to as “Ti/N ratio”or “Ti/N”) is 3.4 or more.

As the Ti/N ratio is 3.4 or more, the effect of improving hardenabilityby B (boron) is effectively exhibited. This point will be described indetail below.

As described above. B is an element that contributes to improving thehardenability of steel.

However, even in a case in which B is contained in steel, B existing inthe form of BN (boron nitride) does not exhibit the function ofimproving hardenability. In this regard, when the Ti/N ratio in thesteel is 3.4 or more, N in the steel is fixed in the form of TiN(titanium nitride). As a result, the formation of BN is suppressed suchthat an effective amount of B that contributes to the improvement ofhardenability is secured. As a result, the effect of improvinghardenability by B (boron) is effectively exhibited.

The Ti/N ratio is preferably 4.0 or more.

The upper limit of the Ti/N ratio depends on the range of Ti content andthe range of N content. In a case in which the N content is 0%, the Ti/Nratio is infinite. The upper limit of the Ti/N ratio is preferably 80.0,more preferably 50.0, and still more preferably 40.0.

<Metallographic Microstructure of Wall Thickness Central Portion in LCross Section at Base Metal 180° Position>

Next, the metallographic microstructure of a wall thickness centralportion in an L cross section at a base metal 180° position will bedescribed.

Here, a wall thickness central portion in an L cross section at a basemetal 180° position is merely a position selected as a representativeposition of a base metal portion.

Therefore, in the electric resistance welded steel pipe of thedisclosure, the metallographic microstructure at a position other thanthe wall thickness central portion in an L cross section at a 180°position in a base metal portion may have the following characteristics.

(Tempered Martensite Structure)

In the electric resistance welded steel pipe of the disclosure, ametallographic microstructure of a wall thickness central portion in anL cross section at a base metal 180° position (i.e., a positiondeviating by 180° in a circumferential direction of the pipe from anelectric resistance welded portion) is a tempered martensite structure.

In the disclosure, the tempered martensite structure means asingle-phase structure substantially consisting of a temperedmartensite.

Here, the single-phase structure substantially consisting of a temperedmartensite means a metallographic microstructure in which a temperedmartensite has an aerial ratio of 80% or more (preferably 90% or more)in when confirmed by a method using a scanning electron microscope (SEM)described later.

Whether or not a metallographic microstructure of a wall thicknesscentral portion in an L cross section at a base metal 180° position is atempered martensite structure is confirmed as follows.

An L cross section (observation face) at a base metal 180° position inan electric resistance welded steel pipe is polished and then etchedwith a initial liquid in accordance with JIS G 0551 (2013). A micrographof the metallographic microstructure of the wall thickness centralportion on the etched L cross section (also hereinafter referred to as“metallographic micrograph”) is taken by a scanning electron microscope(SEM). Metallographic micrographs corresponding to three 3,000-timesvisual fields (one visual field in a range of 40 μm×40 μm) are taken.

The area ratio of the tempered martensite with respect to the entiremetallographic microstructure is calculated based on the metallographicmicrograph taken above (SEM micrograph). In a case in which the arearatio of the tempered martensite is 80% or more, it is determined thatthe tempered martensite structure is formed.

Here, the tempered martensite is a structure in which the lath structureand cementite (iron carbide) can be confirmed on the SEM micrograph, andthe preferential growth direction of cementite is two or more directions(i.e., random).

Meanwhile, the tempered bainite is a structure in which the lathstructure and cementite (iron carbide) can be confirmed on the SEMmicrograph, and the preferential growth direction of cementite isunidirectional.

A structure in which it is difficult to distinguish between the temperedmartensite and the tempered bainite is determined to be the temperedmartensite.

The ferrite is a structure in which the lath structure cannot beconfirmed on the SEM micrograph.

The as-quenched martensite is a structure in which the lath structurecan be confirmed, but cementite cannot be confirmed on the SEMmicrograph.

(Average Aspect Ratio of Prior Austenite Grains)

In the electric resistance welded steel pipe of the disclosure, theaverage aspect ratio of prior austenite grains in the temperedmartensite structure described above is 2.0 or less. As a result, theinner surface cracking resistance is improved as described above.

In a case in which the average aspect ratio of prior austenite grainsexceeds 2.0, the aspect ratio of packet grains in prior austenite grainsalso becomes large, and as a result, the packet grain boundary is easilycontinuous.

For this reason, once a crack is generated on the inner surface of abent portion upon bending forming of the electric resistance weldedsteel pipe, the crack may extend along the packet grain boundary anddevelop into a continuous crack (i.e., inner surface cracking).

The average aspect ratio of prior austenite grains is preferably 1.8 orless, and more preferably 1.6 or less.

The average aspect ratio of prior austenite grains is naturally, bydefinition, 1.0 or more. The average aspect ratio of prior austenitegrains is preferably more than 1.0, and more preferably 1.1 or more.

Here, the average aspect ratio of prior austenite grains means theaverage value of aspect ratios of prior austenite grains.

The aspect ratio of a prior austenite grain means the ratio of the longaxis length to the short axis length (i.e., the long axis length/shortaxis length ratio) when a prior austenite grain is ellipticallyapproximated.

The average aspect ratio of a prior austenite grain is measured asfollows.

The metallographic microstructure of the wall thickness central portionin the L cross section of the base metal portion (specifically a basemetal 180° position in the electric resistance welded steel pipe of thedisclosure) is observed using an SEM-EBSD system (at a magnification of1,000 times), an area surrounded by a grain boundary with a tilt angleof 15° or more is regarded as a prior austenite grain, and the form ofthis prior austenite grain is processed by elliptical approximation. Inthe obtained ellipse, the ratio of a long axis length with respect to ashort axis length (i.e., the long axis length/short axis length ratio)is determined as the aspect ratio of the prior austenite grain.

According to this method, the aspect ratio of every prior austenitegrain in a view range of 200 μm (pipe axis direction)×500 μm (wallthickness direction) is obtained. The arithmetic mean of the obtainedmeasurement values (aspect ratios) is calculated, and the thus obtainedarithmetic mean value is determined as the average aspect ratio of theprior austenite grains.

Here, in general, the above-mentioned long axis length direction isapproximately identical to the pipe axis direction of the electricresistance welded steel pipe (i.e., in the rolling direction duringproduction of a hot-rolled steel sheet as a material), and theabove-mentioned short axis length direction is approximately identicalto the wall thickness direction of the electric resistance welded steelpipe.

It is possible to achieve an average aspect ratio of prior austenitegrains of 2.0 or less as described above by performing rolling in arecrystallization region in the hot rolling process (e.g., by settingthe hot rolling finishing temperature to 920° C. or higher).

(Average Grain Size of Packet Grains)

In the electric resistance welded steel pipe of the disclosure, theaverage grain size of packet grains in the tempered martensite structure(hereinafter, also referred to as “average packet grain size”) ispreferably 10 μm or less.

In a case in which the average packet grain size is 10 μm or less, it ispossible to prevent forming strain due to bending forming fromconcentrating on the coarse packet grains, and the forming strain can bedispersed in each packet grain. As a result, the inner surface crackingresistance is further improved.

The average packet grain size is preferably 8 μm or less.

There is no particular limitation on the lower limit of the averagepacket grain size. From the viewpoint of steel production suitability,the average packet grain size is preferably 3 μm or more, and morepreferably 4 μm or more.

Here, the packet grain is a unit contained in one or more prioraustenite grains, and means a unit composed of a plurality of elongatedcrystals arranged substantially in parallel.

The average packet grain size is measured in the manner described below.

The metallographic microstructure of a wall thickness central portion inan L cross section at a base metal 180° position is observed using anSEM-EBSD system, thereby obtaining EBSD images corresponding to three3,000-times visual fields (one visual field in a range of 40 μm×40 μm).

From the obtained EBSD images, 30 packet grains are arbitrarilyselected.

At this time, a unit composed of a plurality of elongated crystalsarranged substantially in parallel (specifically, a unit surrounded by agrain boundary with a tilt angle of 10° or more) is regarded as a packetgrain.

Next, for the 30 selected packet grains, the equivalent circle diameterof each packet grain is obtained, and the obtained value is used as thegrain size of each packet grain.

Next, the arithmetic mean value of the grain sizes of the packet grainsin 30 packet grains is obtained, and the obtained arithmetic mean valueis defined as the average packet grain size (i.e., the average grainsize of packet grains).

An average grain size of packet grains of 10 μm or less as describedabove can be achieved by allowing the chemical composition of the basemetal portion to include Ti, Al, and Nb in predetermined respectiveamounts or more; performing rolling in a recrystallization region in thehot rolling process (e.g., by setting the hot rolling finishingtemperature to 920° C. or higher); or the like.

(Dislocation Density)

In the electric resistance welded steel pipe of the disclosure, a wallthickness central portion in an L cross section at a 180° position in abase metal has a dislocation density of 2.0×10¹⁴ m⁻² or less.

In a case in which the dislocation density is 2.0×10¹⁴ m⁻² or less, theinner surface cracking resistance is further improved.

From the viewpoint of further improving the inner surface crackingresistance, the dislocation density is preferably 1.9×10¹⁴ m⁻² or less.

There is no particular limitation on the lower limit of the dislocationdensity. Examples of the dislocation density include 0.4×10¹⁴ m⁻² and0.6×10¹⁴ m⁻².

The dislocation density in the disclosure is measured in the mannerdescribed below.

In the wall thickness central portion in an L cross section at a basemetal 180° position, the half-value widths of the (110) plane, (211)plane, and (220) plane are measured by X-ray diffraction, and based onthe measured values, the dislocation density is calculated according tothe Williamson-Hall method (specifically, the method described in ACTAMETALLURGICA Vol. 1, January 1953, pp. 22-31).

The above measurement and calculation are performed at three points inthe wall thickness central portion, and the arithmetic mean value of theobtained three calculated values is defined as the dislocation densityin the disclosure.

The conditions for X-ray diffraction are as follows. As an X-raydiffractometer used for X-ray diffraction, for example, “RINT2200”manufactured by Rigaku Corporation is used

Tube: Mo tube (tube using Mo as a target)

Target output: 50 KV, 40 mA

Slit: 1/2° divergence slit, 1° scattering slit, 0.15 mm receiving slit

Sampling width: 0.010°

Measurement range (2θ): from 34.2° to 36.2°

Maximum number of counts: 3000 or more

<Metallographic Microstructure in Vicinity of Electric Resistance WeldedPortion>

In the electric resistance welded steel pipe of the disclosure, ametallographic microstructure of a vicinity of an electric resistancewelded portion (i.e., an area within a distance corresponding to a wallthickness from the electric resistance welded portion) in a wallthickness central portion in a C cross section includes a temperedmartensite and at least one of a tempered bainite or a ferrite.

FIG. 1 is a schematic cross-sectional view conceptually showing a partof the C cross section of the electric resistance welded steel pipeaccording to an example of the disclosure, which is a figure forexplaining an “area within a distance corresponding to a wall thicknessfrom the electric resistance welded portion in a wall thickness centralportion in a C cross section” (i.e., a vicinity of the electricresistance welded portion).

As shown in FIG. 1, in a wall thickness central portion in a C crosssection, an area V1 within a distance corresponding to wall thickness tfrom an electric resistance welded portion EW1 (i.e., a vicinity of theelectric resistance welded portion) is an area having a length of 2t(i.e., a length twice the wall thickness t) centered on the electricresistance welded portion EW1 on a curve corresponding to the wallthickness central portion in the C cross section. In FIG. 1, the area V1is indicated by a one dot chain line.

The vicinity of the electric resistance welded portion in the wallthickness central portion in the C cross section is merely a positionselected as a representative position of a vicinity of an electricresistance welded portion. Therefore, a metallographic microstructure ofa vicinity of an electric resistance welded portion at a portion otherthan a wall thickness central portion in the C cross section may be ametallographic microstructure including a tempered martensite and atleast one of a tempered bainite or a ferrite.

Whether or not a metallographic microstructure of a vicinity of anelectric resistance welded portion in the wall thickness central portionin the C cross section includes at least one of a tempered bainite or aferrite is confirmed as follows.

A C cross section (observation face) in an electric resistance weldedsteel pipe is polished and then etched with a initial liquid inaccordance with JIS G 0551 (2013). An entire area within a distancecorresponding to a wall thickness from an electric resistance weldedportion (e.g., the area VI described above) in the etched wall thicknesscentral portion in a C cross section is observed by an SEM (at amagnification of 500 times) while scanning the area, thereby confirmingwhether or not at least one of a tempered bainite or a ferrite exists inthe area.

A method of distinguishing a tempered bainite, a ferrite, and a temperedmartensite on an SEM micrograph is as described above.

As described above, the metallographic microstructure in the vicinity ofthe electric resistance welded portion includes at least one of atempered bainite or a ferrite, which indicates that the electricresistance welded steel pipe of the disclosure is an electric resistancewelded steel pipe that has been tempered without quenching after pipemaking.

In contrast to this, in the case of an electric resistance welded steelpipe that has been quenched and tempered after-pipe making, ametallographic microstructure of a vicinity of an electric resistancewelded portion becomes a tempered martensite structure which issubstantially free of a tempered bainite and a ferrite.

<Yield Elongation>

Yield elongation of the electric resistance welded steel pipe of thedisclosure is observed when a tensile test in the pipe axis direction isperformed.

Here, “yield elongation is observed” means that a substantial yieldelongation (specifically, a yield elongation of 0.1% or more) isobserved in a tensile test in the pipe axis direction.

A tensile test in the pipe axis direction for observing the presence orabsence of yield elongation is performed under the same conditions as atensile test in the pipe axis direction for measuring the tensilestrength in the pipe axis direction, which will be described later.

As described above, yield elongation of the electric resistance weldedsteel pipe of the disclosure is observed when a tensile test in the pipeaxis direction is performed, which indicates that the electricresistance welded steel pipe of the disclosure is an electric resistancewelded steel pipe that has been tempered after pipe-making.

For example, in the case of an electric resistance welded steel pipethat has been tempered before but not after pipe-making, yieldelongation is not observed.

<Tensile Strength in Pipe Axis Direction>

The electric resistance welded steel pipe of the disclosure has atensile strength in the pipe axis direction (hereinafter, also simplyreferred to as “tensile strength”) of from 750 to 980 MPa.

As a result of the tensile strength being 750 MPa or more, the strengthof a steel pipe for a torsion beam is ensured. The tensile strength ispreferably 800 MPa or more.

As a result of the tensile strength being 980 MPa or less, inner surfacecracking resistance is improved. The tensile strength is preferably 950MPa or less, and more preferably 900 MPa or less.

The tensile strength of the electric resistance welded steel pipe of thedisclosure is measured in the manner described below.

A JIS 12 tensile test specimen is sampled at a base metal 180° positionof the electric resistance welded steel pipe of the disclosure. Thesampled JIS 12 tensile test specimen is examined by performing a tensiletest in the pipe axis direction in accordance with JIS Z 2241 (2011)(i.e., tensile test with a test direction as a pipe axis direction),thereby measuring the tensile strength in the pipe axis direction. Theobtained results are determined as the tensile strength (i.e., tensilestrength in a pipe axis direction) of the electric resistance weldedsteel pipe of the disclosure.

The outer diameter of the electric resistance welded steel pipe of thedisclosure is not particularly restricted. From the viewpoint of theproduction suitability of an electric resistance welded steel pipe, theouter diameter is preferably from 50 to 150 mm.

The wall thickness of the electric resistance welded steel pipe of thedisclosure is not particularly restricted. From the viewpoint of theproduction suitability of an electric resistance welded steel pipe, thewall thickness of the electric resistance welded steel pipe of thedisclosure is preferably from 2.0 to 4.0 mm.

<Intended Use>

The electric resistance welded steel pipe of the disclosure is used forproducing a torsion beam.

Production of a torsion beam using the electric resistance welded steelpipe of the disclosure is carried out by, for example, processing a partof the electric resistance welded steel pipe of the disclosure bybending forming. Bending forming is performed by, for example, pushing apart of a linear area in parallel with the pipe axis direction of theelectric resistance welded steel pipe of the disclosure in theoutside-to-inside direction of the electric resistance welded steel pipe(e.g., see the bending test illustrated in FIG. 2 described later). As aresult, for example, a torsion beam including a portion having anapproximately V-shaped closed cross section (e.g., see FIG. 3 describedlater) is produced.

Usually, there is a tendency that inner surface cracking is likely tooccur in a case in which the curvature radius R of the inner surface ofa bent portion formed by bending forming is small.

However, according to the electric resistance welded steel pipe of thedisclosure having excellent inner surface cracking resistance, theoccurrence of inner surface cracking is effectively suppressed even insuch a case.

Therefore, the effect of improving inner surface cracking resistanceaccording to the electric resistance welded steel pipe of the disclosureis exerted effectively especially in a case in which the curvatureradius R of the inner surface of a bent portion formed by bendingforming is small.

In other words, the effect of improving inner surface crackingresistance according to the electric resistance welded steel pipe of thedisclosure is exerted effectively especially in a case in which theelectric resistance welded steel pipe of the disclosure is used forproducing a torsion beam including a bent portion having a smallcurvature radius R of the inner surface (e.g., a bent portion having acurvature radius R of the inner surface that is not more than 2 times(preferably 0.7 to 2 times, and more preferably 1 to 2 times) the wallthickness).

<One Example of Production Method>

One example of a method of producing an electric resistance welded steelpipe of the present disclosure is the following production method A.

The production method A includes:

a slab preparation step of preparing a slab having the chemicalcomposition in the disclosure:

a hot rolling step of heating the prepared slab to a slab heatingtemperature of from 1070° C. to 1300° C., and hot rolling the heatedslab under a condition of a hot rolling finishing temperature of 920° C.or more, thereby obtaining a hot-rolled steel sheet,

a cooling step of conducting cooling of the hot-rolled steel sheetobtained in the hot rolling step under conditions that an averagecooling rate from the start of cooling to 200° C. is from 40° C. to 100°C./s to achieve a coiling temperature of 200° C. or less (i.e., acooling end temperature):

a coiling step of coiling the hot-rolled steel sheet after cooling atthe above-mentioned coiling temperature, thereby obtaining a hot coilconfigured from the hot-rolled steel sheet:

a pipe-making step of uncoiling the hot-rolled steel sheet from the hotcoil, roll-forming the uncoiled hot-rolled steel sheet to thereby makean open pipe, and processing abutting portions of the obtained open pipeby electric resistance welding to form an electric resistance weldedportion, thereby obtaining an as-rolled electric resistance welded steelpipe, and

a post-pipe-making tempering step of performing tempering of theas-rolled electric resistance welded steel pipe under conditions of atempering temperature of from 500° C. to 700° C. and a tempering time offrom 1 minute to 120 minutes without quenching.

The production method A may include other steps, if necessary.

The above hot rolling step, cooling step, and coiling step are carriedout using a hot strip mill.

The term “as-rolled electric resistance welded steel pipe” used hereinrefers to an electric resistance welded steel pipe which is notsubjected to heat treatment other than seam heat treatment afterpipe-making. In other words, the expression “as-rolled” in the term“as-rolled electric resistance welded steel pipe “means” as is uponhaving been roll-formed”.

According to the production method A, it is easy to produce the electricresistance welded steel pipe of the disclosure, which means an electricresistance welded steel pipe, in which

a metallographic microstructure of a wall thickness central portion inan L cross section at a base metal 180° position is a temperedmartensite structure, an average aspect ratio of prior austenite grainsin the tempered martensite structure is 2.0 or less,a metallographic microstructure of vicinity of an electric resistancewelded portion in a wall thickness central portion in a C cross sectionincludes a tempered martensite and at least one of a tempered bainite ora ferrite,yield elongation is observed when a tensile test in a pipe axisdirection is performed, and a tensile strength in the pipe axisdirection is from 750 to 980 MPa.

(Slab Preparation Step)

The slab preparation step of the production method A is a step ofpreparing a slab having the above-mentioned chemical composition.

The slab preparation step may be a step of producing a slab or a step ofsimply preparing preliminarily a produced slab.

In the case of slab production, for example, molten steel having theabove-mentioned chemical composition is produced, and slab is producedusing the produced molten steel. At such time, slab may be produced by acontinuous casting method. Alternatively, slab may be produced bypreparing an ingot using molten steel and processing the ingot byslabbing.

(Hot Rolling Step)

The hot rolling step of the production method A is a step of heating theprepared slab to a slab heating temperature of from 1070° C. to 1300°C., and hot rolling the heated slab under a condition of a hot rollingfinishing temperature of 920° C. or more, thereby obtaining hot-rolledsteel sheet.

By heating the slab to a slab heating temperature of from 1070° C. to1300° C., it is possible to solubilize carbide, a nitride compound, anda carbonitride compound which have precipitated in the molten steelsolidification process in steel. As a result, it is possible to improvestrength without deterioration of inner surface cracking resistance. Itis also possible to suppress generation of microvoids duringroll-forming for producing an electric resistance welded steel pipeand/or bending forming on an electric resistance welded steel pipe.

In a case in which the slab heating temperature is 1070° C. or more, itis possible to sufficiently solubilize carbide, a nitride compound, anda carbonitride compound which have precipitated in the molten steelsolidification process in steel.

In a case in which the slab heating temperature is 1300° C. or less, asaustenite grain coarsening is suppressed, it is possible to preventcoarse AlN from precipitating during hot rolling or cooling after hotrolling.

Further, in the hot rolling step, the hot rolling finishing temperaturemeans the end temperature of the finish rolling in hot rolling(sometimes referred to as “finish rolling outlet temperature”).

When the hot rolling finishing temperature is 920° C. or more, thismeans that hot rolling is performed in a recrystallization region,rather than being performed in a non-recrystallization region. As aresult, an electric resistance welded steel pipe to be obtained islikely to achieve an average aspect ratio of prior austenite grains of2.0 or less.

Further, when the hot rolling finishing temperature is 920° C. or more,it contributes to refining of prior austenite grains and also torefining of packet grains in prior austenite grains. Therefore, due tothe hot rolling finishing temperature being 920° C. or more, it is alsoeasy to achieve an average packet grain size of 10 μm or less.

The upper limit of the hot rolling finishing temperature isappropriately set, but the upper limit is preferably 1000° C. from theviewpoint of further suppressing the coarsening of austenite grains.

(Cooling Step)

In the production method A, a cooling step is a step of conductingcooling of the hot-rolled steel sheet obtained in the hot rolling stepunder conditions that an average cooling rate from the start of coolingto 200° C. is from 40° C. to 100° C./s to achieve a coiling temperatureof 200° C. or less.

In the cooling step, by conducting cooling of the hot-rolled steel sheetobtained in the hot rolling step under conditions that an averagecooling rate from the start of cooling to 200° C. is 40° C./s or more toachieve a coiling temperature of 200° C. or less (i.e., a cooling endtemperature), an as-quenched martensite structure is formed as ametallographic microstructure of the hot-rolled steel sheet. In otherwords, by this cooling step, the hot-rolled steel sheet is substantiallyquenched.

In the post-pipe-making tempering step described later, the as-quenchedmartensite structure of the base metal portion (e.g., at the base metal180° position) generated in this cooling step is tempered such that atempered martensite structure is formed.

Meanwhile, in a case in which the average cooling rate from the start ofcooling to 200° C. is 100° C./s or less, it is easy to control thecooling termination temperature. In addition, in a case in which theaverage cooling rate is 100° C./s or less, the difference in the coolingrate between the surface of the hot-rolled steel sheet and the internalportion in a wall thickness direction (e.g., a wall thickness centralportion) becomes small such that stability of the material of thehot-rolled steel is more excellent.

(Coiling Step)

The coiling step of the production method A is a step of coiling thehot-rolled steel sheet after cooling at the above-mentioned coilingtemperature, thereby obtaining a hot coil configured from the hot-rolledsteel sheet.

(Pipe-Making Step)

The pipe-making step of the production method A is a step of uncoilingthe hot-rolled steel sheet from the hot coil, roll-forming the uncoiledhot-rolled steel sheet to thereby make an open pipe, and subjectingabutting portions of the obtained open pipe to electric resistancewelding to form an electric resistance welded portion, thereby obtainingan as-rolled electric resistance welded steel pipe.

The pipe-making step can be carried out in accordance with a knownmethod.

The pipe-making step may optionally include: applying seam heattreatment to an electric resistance welded portion after forming theelectric resistance welded portion; reducing the outer diameter of theas-rolled electric resistance welded steel pipe by a sizer after formingthe electric resistance welded portion (after seam heat treatment whenconducting the seam heat treatment described above), and the like.

(Post-Pipe-Making Tempering Step)

The post-pipe-making tempering step of the production method A is a stepof performing tempering of the as-rolled electric resistance weldedsteel pipe under conditions of a tempering temperature of from 500° C.to 700° C. and a tempering time of from 1 minute to 120 minutes withoutquenching.

As a result of the post-pipe-making tempering step, the as-quenchedmartensite structure of the base metal portion (e.g., at the base metal180° position) is effectively tempered such that a tempered martensitestructure is formed.

In a vicinity of the electric resistance welded portion, the as-quenchedmartensite structure is reverse-transformed into an austenite anddisappears due to electric resistance welding. After electric resistancewelding, the austenite is cooled and then tempered after pipe-makingsuch that the above-described metallographic microstructure including atempered martensite and at least one of a tempered bainite or a ferriteis formed.

In the post-pipe-making tempering step, as the tempering conditionsinclude a tempering temperature of 500° C. or more and a tempering timeof 1 minute or more, a tempered martensite structure can be effectivelyformed in the base metal portion.

Further, tempering under such conditions allows effectively reducingpipe-making strain, thereby effectively reducing the dislocationdensity.

In addition, in the post-pipe-making tempering step, as the temperingconditions include a tempering temperature of 700° C. or less and atempering time of 120 minutes or less, strength reduction due toexcessive tempering is suppressed such that a tensile strength of 750MPa or more can be easily achieved.

In the post-pipe-making tempering step, tempering under the aboveconditions is performed without quenching.

Quenching as used herein refers to an operation in which a steel pipe isheat-treated at a temperature of the A3 point or higher and is rapidlycooled.

The A3 point means a temperature at which the transformation toaustenite is completed during heating, and it depends on the chemicalcomposition of the steel pipe. In the chemical composition in thedisclosure, the A3 point does not fall below 700° C., and therefore, theabove-described tempering does not correspond to quenching.

In the post-pipe-making tempering step, the cooling after the temperingis not particularly limited, and may be slow cooling (e.g., air cooling)or rapid cooling (e.g., water cooling).

Each of the steps of the production method A described above does notaffect the chemical composition of steel.

Accordingly, the chemical composition of the base metal portion of anelectric resistance welded steel pipe produced by the production methodA can be considered to be identical to the chemical composition of a rawmaterial (molten steel or slab).

EXAMPLES

Hereinafter, the present invention will be described in more detail withreference to Examples below, but the invention is not limited to theseExamples.

Examples 1 to 10 and Comparative Examples 1 to 29 <Production ofElectric Resistance Welded Steel Pipe>

An electric resistance welded steel pipe was obtained in each ofExamples 1 to 10 according to the above-mentioned production method A.

In addition, the chemical composition or the production conditions werechanged in the electric resistance welded steel pipes of the respectiveExamples to thereby obtain the electric resistance welded steel pipes ofComparative Examples 1 to 29.

Details are described below.

Molten steels (steels A to Z, AA, AB, and AC) having the chemicalcompositions set forth in Table 1 were produced in a furnace, and slabseach having a thickness of 250 mm were prepared by casting (the slabpreparation step).

In Table 1, the value shown in the column of each element is the percent(%) by mass of each element.

The balance excluding the elements set forth in Table 1 is Fe andimpurities.

In Table 1, REM in steel H is La.

V_(c90) in Table 1 is V_(c90) defined by Formula (i) described above.

Each underline in Tables 1 to 3 indicates a value that does not fallwithin the range of the disclosure.

Each slab obtained above was heated to the slab heating temperature setforth in Table 2 or 3, and then processed by hot-rolling under thecondition of the hot-rolling finishing temperature set forth in Table 2or 3, thereby obtaining a hot-rolled steel sheet (the hot rolling step).

Each hot-rolled steel sheet obtained in the hot rolling step wassubjected to cooling at an average cooling rate set forth in Table 2 or3 to achieve a coiling temperature set forth in Table 2 or 3 (i.e., acooling end temperature) (cooling step).

Then, each hot-rolled steel sheet was coiled at the coiling temperatureset forth in Table 2 or 3, thereby obtaining a hot coil configured fromthe hot-rolled steel sheet having a sheet thickness of 3.0 mm (coilingstep).

The above hot rolling step, cooling step, and coiling step were carriedout using a hot strip mill.

In Examples 1 to 10 and Comparative Examples 1 to 24 and 26 to 29, apipe-making step of uncoiling the hot-rolled steel sheet from the hotcoil, roll-forming the uncoiled hot-rolled steel sheet to thereby makean open pipe, and processing abutting portions of the obtained open pipeby electric resistance welding to form an electric resistance weldedportion, and then reducing the diameter by a sizer, thereby obtaining anas-rolled electric resistance welded steel pipe having an outer diameterof 90 mm and a wall thickness of 3.0 mm.

In Comparative Example 25, the hot-rolled steel sheet was uncoiled fromthe hot coil, and the uncoiled hot-rolled steel sheet was temperedbefore pipe-making under the conditions set forth in Table 3 (temperingtemperature and tempering time), and then the hot-rolled steel sheet wascoiled again. The coiled hot-rolled steel sheet was uncoiled again, andthe uncoiled hot-rolled steel sheet was used to thereby obtain anas-rolled electric resistance welded steel pipe having an outer diameterof 90 mm and a wall thickness of 3.0 mm in the same manner as in Example1.

In Examples 1 to 10 and Comparative Examples 1 to 24 and 26 to 29, theas-rolled electric resistance welded steel pipe was tempered afterpipe-making under the conditions set forth in Tables 2 and 3 (temperingtemperature and tempering time), and then air-cooled to thereby obtainan electric resistance welded steel pipe having an outer diameter of 90mm and a wall thickness of 3.0 mm (post-pipe-making tempering step).

In Comparative Example 25, the as-rolled electric resistance weldedsteel pipe was not tempered after pipe-making.

<Observation of L Cross Section at Base Metal 180° Position>

An L cross section at a base metal 180° position in the electricresistance welded steel pipe obtained above (as-rolled electricresistance welded steel pipe in Comparative Example 25; the same applieshereinafter) was observed, and the following confirmation andmeasurement were carried out.

(Metallographic Microstructure of Wall Thickness Central Portion)

The metallographic microstructure of a wall thickness central portion inthe L cross section at the base metal 180° position was confirmed by themethod described above.

The results are set forth in Tables 2 and 3.

In Tables 2 and 3,

“TM” means tempered martensite structure,

“TM+TB” means a dual-phase structure consisting of a tempered martensiteand a tempered bainite,

“F+TB” means a dual-phase structure consisting of a ferrite and atempered bainite, and

“TB” means tempered bainite structure.

(Average Aspect Ratio of Prior Austenite Grains}

The average aspect ratio of prior austenite grains (referred to as“average aspect ratio of prior γ grains” in Tables 2 and 3) of the wallthickness central portion in the L cross section was measured by themethod described above.

The results are set forth in Tables 2 and 3.

(Average Packet Grain)

The average packet grain at the wall thickness central portion in the Lcross section was measured by the method described above.

The results are set forth in Tables 2 and 3.

(Dislocation Density)

The dislocation density of the wall thickness central portion in the Lcross section was measured by the method described above. As an X-raydiffractometer used for X-ray diffraction, “RINT2200” manufactured byRigaku Corporation was used.

The results are set forth in Tables 2 and 3.

<Observation of Vicinity of Electric Resistance Welded Portion in WallThickness Central Portion in C Cross Section>

A vicinity of an electric resistance welded portion in a wall thicknesscentral portion in a C cross section (i.e., an area within a distancecorresponding to a wall thickness from the electric resistance weldedportion) of each of the electric resistance welded steel pipes ofExamples 1 to 10 was observed by the method described above.

As a result, in each of Examples 1 to 10, a metallographicmicrostructure of the vicinity of the electric resistance welded portionwas confirmed to include a tempered martensite and at least one of atempered bainite or a ferrite.

<Measurement of Tensile Strength in Pipe Axis Direction>

The tensile strength in a pipe axis direction of each electricresistance welded steel pipe (hereinafter, simply referred to as“tensile strength”) was measured by the method described above.

The results are set forth in Tables 2 and 3.

<Presence or Absence of Yield Elongation>

The presence or absence of yield elongation was confirmed in a tensiletest in the pipe axis direction for measuring tensile strength.

In a case in which a yield elongation of 0.1% or more was observed, itwas determined that the yield elongation was “present,” and in a case inwhich a yield elongation of 0.1% or more was not observed, it wasdetermined that the yield elongation was “absent.”

<Bending Test (Evaluation of Inner Surface Crack Depth)>

Each electric resistance welded steel pipe was examined by a bendingtest simulating production of a torsion beam, and the inner surfacecrack depth was evaluated. Details are described below.

FIG. 2 is a schematic view showing the outline of the bending test.

As set forth in FIG. 2, an electric resistance welded steel pipe 100A,which is any of the electric resistance welded steel pipes of Examplesand Comparative Examples, a lower die 10 having a V-shaped notchportion, and a punch 12 having a tip having an approximately triangularcross section were prepared.

Here, both an angle θ1 of a trough of a notch portion of a die 10 and anangle θ2 of a tip of a punch 12 were set to 60°.

In this bending test, an electric resistance welded steel pipe 100A wasplaced in the notch portion of the lower die 10, the punch 12 was pushedinto the placed electric resistance welded steel pipe 100A in thedirection of arrow P, thereby conducting bending forming of the electricresistance welded steel pipe 100A. As a result, the electric resistancewelded steel pipe 100A was partially bent in a direction perpendicularto the pipe axis direction L of the electric resistance welded steelpipe 100A, thereby forming a structure 100B having an approximatelyV-shaped closed cross section set forth in FIG. 3.

Here, the pipe axis direction L of the electric resistance welded steelpipe 100A corresponds to the rolling direction during production of ahot-rolled steel sheet as a material for the electric resistance weldedsteel pipe 10A.

FIG. 3 is a schematic cross-sectional view schematically showing a crosssection of a structure 100B obtained by processing an electricresistance welded steel pipe 100A by bending forming in the bendingtest. The cross section of the structure 100B set forth in FIG. 3 is across section perpendicular to the longitudinal direction of thestructure 100B, which corresponds to a C cross section of an electricresistance welded steel pipe before bending forming (i.e., a crosssection perpendicular to the pipe axis direction L).

As set forth in FIGS. 2 and 3, a structure 100B having an approximatelyV-shaped closed cross section was formed by processing an electricresistance welded steel pipe 100A by bending forming. Here, the amountof pushing by the punch 12 was adjusted such that the curvature radius Rof an inner surface 102B was set to 4 mm at one end portion 101B (bentportion) of a closed cross section of the structure 100B. The curvatureradius R for the other end portion of a closed cross section of thestructure 100B was also adjusted to 4 mm.

The inner surface 102B in the cross section of the one end portion 101B(specifically, a cross section corresponding to FIG. 3) and the vicinitythereof were observed by an SEM at a magnification of 1,000 times,thereby measuring the depth of an inner surface crack (hereinafter alsoreferred to as “inner surface crack depth”).

Here, the inner surface crack depth was determined in the mannerdescribed below.

The presence or absence of an inner surface cracks was confirmed byobserving the inner surface 102B in the cross section of the one endportion 101B and the vicinity thereof by SEM. In a case in which innersurface cracks were present, the linear distance between the origin andthe end of a crack was determined for each inner surface crack, therebydetermining the depth of each inner surface crack. The maximum valueamong the depths of individual inner surface cracks was designated asthe “inner surface crack depth” in Examples or Comparative Examples. Ina case in which no inner surface cracks were present, the “inner surfacecrack depth” in Examples or Comparative Examples was determined to be “0μm.”

The obtained inner surface crack depths are set forth in Tables 2 and 3.

In the evaluation of the inner surface crack depth, as the inner surfacecrack depth decreases, the inner surface cracking resistance becomesmore excellent. When the inner surface crack depth is 0 μm, it meansthat no inner surface cracks are formed, indicating that remarkablyexcellent inner surface cracking resistance is achieved.

TABLE 1 Steel C Si Mn P S Ti Al Nb N B Cu Ni A 0.10 0.25 1.20 0.0150.004 0.020 0.024 0.034 0.005 0.0015 B 0.08 0.03 0.86 0.017 0.004 0.1030.031 0.032 0.005 0.0005 C 0.12 1.20 2.50 0.015 0.004 0.076 0.025 0.0400.001 0.0050 D 0.18 0.20 0.50 0.010 0.004 0.010 0.024 0.031 0.002 0.0010E 0.06 0.24 0.81 0.030 0.003 0.030 0.028 0.022 0.005 0.0020 F 0.30 0.270.92 0.010 0.003 0.200 0.500 0.010 0.005 0.0015 0.098 G 0.22 0.77 0.300.019 0.004 0.055 0.026 0.036 0.004 0.0030 H 0.14 0.27 0.78 0.014 0.0040.083 0.032 0.040 0.006 0.0018 0.176 I 0.05 0.28 1.00 0.016 0.010 0.0200.005 0.034 0.005 0.0015 J 0.11 0.24 1.22 0.020 0.004 0.040 0.026 0.0320.005 0.0012 0.134 K 0.04 0.25 1.20 0.015 0.004 0.020 0.024 0.034 0.0050.0015 L 0.31 0.25 1.20 0.015 0.004 0.020 0.024 0.034 0.005 0.0015 M0.10 0.02 1.20 0.015 0.004 0.020 0.024 0.034 0.005 0.0015 N 0.10 1.221.20 0.015 0.004 0.020 0.024 0.034 0.005 0.0015 O 0.10 0.25 0.28 0.0150.004 0.020 0.024 0.034 0.005 0.0015 P 0.10 0.25 2.53 0.015 0.004 0.0200.024 0.034 0.005 0.0015 Q 0.10 0.25 1.20 0.031 0.004 0.020 0.024 0.0340.005 0.0015 R 0.10 0.25 1.20 0.015 0.011 0.020 0.024 0.034 0.005 0.0015S 0.10 0.25 1.20 0.015 0.004 0.009 0.024 0.034 0.005 0.0015 T 0.10 0.251.20 0.015 0.004 0.210 0.024 0.034 0.005 0.0015 U 0.10 0.25 1.20 0.0150.004 0.020 0.004 0.034 0.005 0.0015 V 0.10 0.25 1.20 0.015 0.004 0.0200.510 0.034 0.005 0.0015 W 0.10 0.25 1.20 0.015 0.004 0.020 0.024 0.0090.005 0.0015 X 0.10 0.25 1.20 0.015 0.004 0.020 0.024 0.041 0.005 0.0015Y 0.10 0.25 1.20 0.015 0.004 0.020 0.024 0.034 0.007 0.0015 Z 0.10 0.251.20 0.015 0.004 0.020 0.024 0.034 0.005 0.0004 AA 0.10 0.25 1.20 0.0150.004 0.020 0.024 0.034 0.005 0.0051 AB 0.10 0.25 0.50 0.015 0.004 0.0200.024 0.034 0.005 0.0015 AC 0.10 0.25 1.20 0.015 0.004 0.012 0.024 0.0340.005 0.0015 Steel Cr Mo V W Ca Mg Zr REM V_(c90) Ti/N A 0.0032 57.9 4.0B 132.9 20.8 C 0.0025 2.9 75.6 D 0.0034 138.2 5.0 E 0.0018 138.2 6.4 F0.08 32.3 36.6 G 0.12 0.0026 92.6 13.7 H 0.36 0.010 0.0011 0.0002 0.001324.7 15.0 I 0.010 100.3 4.4 J 0.0050 0.0004 48.5 8.6 K 0.0034 76.6 4.0 L21.7 4.0 M 0.0036 67.8 4.0 N 29.6 4.0 O 283.5 4.0 P 0.0023 5.8 4.0 Q57.9 4.0 R 0.0018 57.9 4.0 S 57.9 1.8 T 57.9 42.0 U 0.0025 57.9 4.0 V57.9 4.0 W 0.0016 57.9 4.0 X 57.9 4.0 Y 0.0012 57.9 2.9 Z 57.9 4.0 AA57.9 4.0 AB 0.0032 193.9 4.0 AC 0.0032 57.9 2.4

TABLE 2 Hot-rolled steel sheet Tempering Tempering Electric resistancewelded steel pipe production condition before after Wall thicknesscentral portion in Hot pipe- pipe- an L cross section at base metalroll- making making 180° position Pres- Slab ing Tem- Tem- Dis- enceheat- finish- Aver- Coil- per- per- Aver- Aver- loca- or ab- Inner inging age ing ing Tem- ing Tem- age age tion sence sur- tem- tem- cool-tem- tem- per- tem- per- Metallo aspect packet den- of face pera- pera-ing pera- pera- ing pera- ing graphic ratio of grain sity Tensile yieldcrack ture ture rate ture ture time ture time micro prior γ size (×10¹⁴strength elon- depth Steel (° C.) (° C.) (° C./s) (° C.) (° C.) (min) (°C.) (min) structure grains (μm) m⁻²) (MPa) gation (μm) Exam- A 1200 94050 50 — — 600 60 TM 1.5 5 0.8 780 Present  0 ple 1 Exam- B 1200 940 100 200  — — 600 60 TM 1.5 5 0.7 850 Present  0 ple 2 Exam- C 1200 940 50 50— — 500 60 TM 1.5 4 1.9 850 Present  0 ple 3 Exam- D 1200 920 50 50 — —560 120  TM 1.9 10  0.9 750 Present  0 ple 4 Exam- E 1200 940 40 50 — —600 60 TM 1.5 5 0.8 800 Present  0 ple 5 Exam- F 1200 940 50 50 — — 700 1 TM 1.5 5 1.2 900 Present  0 ple 6 Exam- G 1200 1000  50 50 — — 600 60TM 1.2 4 0.8 890 Present  0 ple 7 Exam- H 1200 940 70 50 — — 620 60 TM1.5 5 0.6 890 Present  0 ple 8 Exam- I 1200 940 80 50 — — 600 18 TM 1.55 0.9 760 Present  0 ple 9 Exam- J 1200 940 50 50 — — 600 60 TM 1.5 50.8 850 Present  0 ple 10 Com- K 1200 940 50 50 — — 600 60 TM 1.5 5 0.8560 Present  0 parative Exam- ple 1 Com- L 1200 940 50 50 — — 600 60 TM1.5 5 0.9 1010  Present 30 parative Exam- ple 2 Com- M 1200 940 50 50 —— 600 60 TM 1.5 5 0.8 780 Present 10 parative Exam- ple 3 Com- N 1200940 50 50 — — 600 60 TM 1.5 5 0.8 780 Present 10 parative Exam- ple 4Com- O 1200 940 50 50 — — 600 60 TM + TB 1.5 5 0.6 700 Present  5parative Exam- ple 5 Com- P 1200 940 50 50 — — 600 60 TM 1.5 5 1.0 990Present 20 parative Exam- ple 6 Com- Q 1200 940 50 50 — — 600 60 TM 1.55 0.8 780 Present 10 parative Exam- ple 7 Com- R 1200 940 50 50 — — 60060 TM 1.5 5 0.8 780 Present 10 parative Exam- ple 8 Com- S 1200 940 5050 — — 600 60 F + TB 1.5 13  0.6 570 Present 15 parative Exam- ple 9Com- T 1200 940 50 50 — — 600 60 TM 1.5 5 0.8 780 Present 10 parativeExam- ple 10 Com- U 1200 940 50 50 — — 600 60 TM 1.5 12  0.8 780 Present10 parative Exam- ple 11 Com- V 1200 940 50 50 — — 600 60 TM 1.5 5 0.8780 Present 10 parative Exam- ple 12 Com- W 1200 940 50 50 — — 600 60 TM1.5 11  0.8 780 Present 10 parative Exam- ple 13 Com- X 1200 940 50 50 —— 600 60 TM 1.5 5 0.8 780 Present 15 parative Exam- ple 14 Com- Y 1200940 50 50 — — 600 60 F + TB 1.5 5 0.8 700 Present 10 parative Exam- ple15 Com- Z 1200 940 50 50 — — 600 60 F + TB 1.5 5 0.6 580 Present 15parative Exam- ple 16 Com- AA 1200 940 50 50 — — 600 60 TM + TB 1.5 50.8 650 Present 20 parative Exam- ple 17

TABLE 3 Hot-rolled steel sheet Tempering Tempering Electric resistancewelded steel pipe production condition before after Wall thicknesscentral portion in Hot pipe- pipe- an L cross section at base metalroll- making making 180° position Pres- Slab ing Tem- Tem- Dis- enceheat- finish- Aver- Coil- per- per- Aver- Aver- loca- or ab- Inner inging age ing ing Tem- ing Tem- age age tion sence sur- tem- tem- cool-tem- tem- per- tem- per- Metallo aspect packet den- of face pera- pera-ing pera- pera- ing pera- ing graphic ratio of grain sity Tensile yieldcrack ture ture rate ture ture time ture time micro prior γ size (×10¹⁴strength elon- depth Steel (° C.) (° C.) (° C./s) (° C.) (° C.) (min) (°C.) (min) structure grains (μm) m⁻²) (MPa) gation (μm) Com- A 1200 94050 50 — — 600 60 TM 2.1 11 0.8 780 Present  5 parative Exam- ple 18 Com-A 1200 940 39 50 — — 600 60 TM + TB 1.5  5 0.8 740 Present  5 parativeExam- ple 19 Com- A 1200 940 50 500  — — 600 60 F + TB 1.5  5 0.6 580Present 15 parative Exam- ple 20 Com- A 1200 940 50 50 — — 310 60 TM 1.5 5 5.9 870 Absent 20 parative Exam- ple 21 Com- A 1200 940 50 50 — — 74060 TM 1.5  5 0.4 560 Present  0 parative Exam- ple 22 Com- A 1200 940 5050 — — 600 0.9 TM 1.5  5 2.1 800 Absent  5 parative Exam- ple 23 Com- A1200 940 50 50 — — 600 150 TM 1.5  5 0.4 570 Present  0 parative Exam-ple 24 Com- A 1200 940 50 50 600 60 — — TM 1.5  5 7.1 840 Absent 60parative Exam- ple 25 Com- A 1200 850 50 50 — — 600 60 TM 5.2 13 0.9 790Present 15 parative Exam- ple 26 Com- A 1200 940 50 400  — — 600 60 TB1.5  5 0.6 580 Present 10 parative Exam- ple 27 Com- AB 1200 940 50 50 —— 600 60 TM + TB 1.5  5 0.8 610 Present 15 parative Exam- ple 28 Com- AC1200 940 50 50 — — 600 60 TM + TB 1.5  5 0.8 630 Present 10 parativeExam- ple 29

As shown in Table 2, excellent tensile strength in a range of from 750MPa to 980 MPa and excellent inner surface cracking resistance wereconfirmed for the electric resistance welded steel pipes of Examples 1to 10, each of which has the chemical composition in the disclosure, andin which a metallographic microstructure of a wall thickness centralportion in an L cross section at a base metal 180° position is atempered martensite structure, an average aspect ratio of prioraustenite grains in the tempered martensite structure is 2.0 or less,and yield elongation is observed when a tensile test in the pipe axisdirection is performed.

The results of the Comparative Examples shown in Table 2 with respect tothe above Examples are as follows.

In Comparative Example 1 in which the C content was excessively small,the tensile strength was insufficient.

In Comparative Example 2 in which the C content was excessively large,the tensile strength became excessive and the inner surface crackingresistance deteriorated.

In Comparative Example 3 in which the Si content was excessively small,the inner surface cracking resistance deteriorated. This is thought tobe because deoxidation was insufficient, and thus coarse Fe oxide wasgenerated.

In Comparative Example 4 in which the Si content was excessively large,inner surface cracking resistance deteriorated. This is thought to bebecause an inclusion such as SiO₂ was generated, thereby facilitatinggeneration of microvoids starting from the inclusion during roll-formingfor producing an electric resistance welded steel pipe and/or bendingforming on an electric resistance welded steel pipe.

In Comparative Example 5 in which the Mn content was excessively small,the metallographic microstructure of a wall thickness central portion inan L cross section at a base metal 180° position was a dual-phasestructure consisting of a tempered martensite and a tempered bainite butnot a tempered martensite structure, and thus the tensile strengthbecame insufficient, and the inner surface cracking resistancedeteriorated.

In Comparative Example 6 in which the Mn content was excessively large,the tensile strength became excessive, and the inner surface crackingresistance deteriorated.

In Comparative Example 7 in which the P content was excessively large,the inner surface cracking resistance deteriorated. This is thought tobe because P was concentrated at the packet grain boundary.

In Comparative Example 8 in which the S content was excessively large,the inner surface cracking resistance deteriorated. This is thought tobe because coarse MnS was generated.

In Comparative Example 9 in which the Ti content was excessively small,Ti/N was less than 3.4, and the metallographic microstructure of a wallthickness central portion in an L cross section at a base metal 180°position was a dual-phase structure consisting of a ferrite and atempered bainite but not a tempered martensite structure, and thus thetensile strength became insufficient and the inner surface crackingresistance deteriorated. This is thought to be because N could not befixed in the form of TiN, and BN was generated, and as a result, theeffect of improving hardenability by B became insufficient.

In Comparative Example 10 in which the Ti content was excessively large,the inner surface cracking resistance deteriorated. This is thought tobe because coarse TiC and/or TiN precipitated

in Comparative Example 11 in which the Al content was excessively small,the inner surface cracking resistance deteriorated. This is thought tobe because prior austenite grains became coarse, and the packet grainsin prior austenite grains also became coarse.

In Comparative Example 12 in which the Al content was excessively large,the inner surface cracking resistance deteriorated. This is thought tobe because coarse AlN was generated.

In Comparative Example 13 in which the Nb content was excessively small,the inner surface cracking resistance deteriorated. This is thought tobe because prior austenite grains became coarse, and the packet grainsin prior austenite grains also became coarse.

In Comparative Example 14 in which the Nb content was excessively large,the inner surface cracking resistance deteriorated. This is thought tobe because coarse NbC was generated.

In Comparative Example 15 in which the N content was excessively large,the inner surface cracking resistance deteriorated. This is thought tobe because coarse AlN was generated. Also in Comparative Example 15,Ti/N was less than 3.4, and the metallographic microstructure of a wallthickness central portion in an L cross section at a base metal 180°position was a dual-phase structure consisting of a ferrite and atempered bainite, and thus the tensile strength became insufficient.This is thought to be because N could not be fixed in the form of TiN,and BN was generated, and as a result, the effect of improvinghardenability by B became insufficient.

In Comparative Example 16 in which the B content was excessively small,the metallographic microstructure of a wall thickness central portion inan L cross section at a base metal 180° position was a dual-phasestructure consisting of a ferrite and a tempered bainite but not atempered martensite structure, and thus the tensile strength becameinsufficient, and the inner surface cracking resistance deteriorated.This is thought to be because the B content was excessively small, andthus hardenability became insufficient.

In Comparative Example 17 in which the B content was excessively large,the metallographic microstructure of a wall thickness central portion inan L cross section at a base metal 180° position was a dual-phasestructure consisting of a tempered martensite and a tempered bainite butnot a tempered martensite structure, and thus the tensile strengthbecame insufficient, and the inner surface cracking resistancedeteriorated. This is thought to be because since B aggregated and/orprecipitated, the solid solution B segregated at the austenite grainboundary decreased, and thus hardenability decreased.

The results of the Comparative Examples shown in Table 3 with respect tothe above Examples are as follows.

In Comparative Example 18, which had the chemical composition of thedisclosure, but the hot rolling finishing temperature was excessivelylow, the average aspect ratio of prior austenite grains was more than2.0, and the inner surface cracking resistance deteriorated.

In Comparative Example 19, which had the chemical composition ofdisclosure, but in which the cooling rate when cooling the hot-rolledsteel sheet was excessively slow, the metallographic microstructure of awall thickness central portion in an L cross section at a base metal180° position was a dual-phase structure consisting of a temperedmartensite and a tempered bainite but not a tempered martensitestructure, and thus the tensile strength became insufficient, and theinner surface cracking resistance deteriorated.

In Comparative Example 20, which had the chemical composition ofdisclosure, but in which the coiling temperature when coiling thehot-rolled steel sheet (i.e., the cooling end temperature) wasexcessively high, the metallographic microstructure of a wall thicknesscentral portion in an L cross section at a base metal 180° position wasa dual-phase structure consisting of a ferrite and a tempered bainitebut not a tempered martensite structure, and thus the tensile strengthbecame insufficient, and the inner surface cracking resistancedeteriorated.

In Comparative Example 21, which had the chemical composition ofdisclosure, but in which the tempering temperature in tempering afterpipe-making was excessively low, yield elongation was not observed, andthe inner surface cracking resistance deteriorated.

In Comparative Example 22, which had the chemical composition ofdisclosure, but in which the tempering temperature in tempering afterpipe-making was excessively high, the tensile strength was insufficient.

In Comparative Example 23, which had the chemical composition ofdisclosure, but in which the tempering time in tempering afterpipe-making was excessively short, yield elongation was not observed,and the inner surface cracking resistance deteriorated.

In Comparative Example 24, which had the chemical composition ofdisclosure, but in which the tempering time in tempering afterpipe-making was excessively long, the tensile strength was insufficient.

In Comparative Example 25, which had the chemical composition ofdisclosure, but in which tempering was performed before but not afterpipe-making, yield elongation was not observed, and the inner surfacecracking resistance deteriorated.

In Comparative Example 26, which had the chemical composition of thedisclosure, but the hot rolling finishing temperature was excessivelylow, the average aspect ratio of prior austenite grains was more than2.0, and the inner surface cracking resistance deteriorated.

In Comparative Example 27, which had the chemical composition ofdisclosure, but in which the coiling temperature when coiling thehot-rolled steel sheet (i.e., the cooling end temperature) wasexcessively high, the metallographic microstructure of a wall thicknesscentral portion in an L cross section at a base metal 180° position wasa tempered bainite structure but not a tempered martensite structure,and thus the tensile strength became insufficient, and the inner surfacecracking resistance deteriorated.

In Comparative Example 28 in which the content of each element in thechemical composition of the disclosure is appropriate, but V_(c90)exceeds 150, the metallographic microstructure of a wall thicknesscentral portion in an L cross section at a base metal 180° position wasa dual-phase structure consisting of a tempered martensite and atempered bainite but not a tempered martensite structure, and thus thetensile strength became insufficient, and the inner surface crackingresistance deteriorated.

In Comparative Example 29 in which the content of each element in thechemical composition of the disclosure is appropriate, but Ti/N is lessthan 3.4, the metallographic microstructure of a wall thickness centralportion in an L cross section at a base metal 180° position was adual-phase structure consisting of a tempered martensite and a temperedbainite, and thus the tensile strength became insufficient. This isthought to be because N could not be fixed in the form of TiN, and BNwas generated, and as a result, the effect of improving hardenability byB became insufficient.

1. An electric resistance welded steel pipe for a torsion beam, thesteel pipe comprising a base metal portion and an electric resistancewelded portion, wherein a chemical composition of the base metal portionconsists of, in terms of % by mass: 0.05 to 0.30% of C, 0.03 to 1.20% ofSi, 0.30 to 2.50% of Mn, 0 to 0.030% of P, 0 to 0.010% of S, 0.010 to0.200% of Ti, 0.005 to 0.500% of Al, 0.010 to 0.040% of Nb, 0 to 0.006%of N, 0.0005 to 0.0050% of B, 0 to 1.000% of Cu, 0 to 1.000% of Ni, 0 to1.00% of Cr, 0 to 0.50% of Mo, 0 to 0.200% of V, 0 to 0.100% of W, 0 to0.0200% of Ca, 0 to 0.0200% of Mg, 0 to 0.0200% of Zr, 0 to 0.0200% ofREM, and, a balance consisting of Fe and impurities, wherein: V_(c90),defined by the following Formula (i), is from 2 to 150, a mass ratio ofTi content to N content is 3.4 or more, a metallographic microstructureof a wall thickness central portion is a tempered martensite structure,and an average aspect ratio of prior austenite grains in the temperedmartensite structure is 2.0 or less, in an L cross section at a positiondeviating by 180° in a circumferential direction of the pipe from theelectric resistance welded portion, a metallographic microstructure ofan area within a distance corresponding to a wall thickness from theelectric resistance welded portion in a wall thickness central portionin a C cross section includes a tempered martensite and at least one ofa tempered bainite or a ferrite, yield elongation is observed when atensile test in a pipe axis direction is performed, and a tensilestrength in the pipe axis direction is from 750 to 980 MPa:log V_(c90)=2.94−0.75βa  Formula (i)βa=2.7C+0.4Si+Mn+0.45Ni+0.8Cr+2Mo  Formula (ii) wherein, in Formula (i),sa represents a value defined by Formula (ii), and in Formula (ii),element symbols represent % by mass of respective elements.
 2. Theelectric resistance welded steel pipe for a torsion beam according toclaim 1, wherein the chemical composition of the base metal portioncontains, in terms of % by mass, at least one selected from the groupconsisting of: more than 0% but equal to or less than 1.000% of Cu, morethan 0% but equal to or less than 1.000% of Ni, more than 0% but equalto or less than 1.00% of Cr, more than 0% but equal to or less than0.50% of Mo, more than 0% but equal to or less than 0.200% of V, morethan 0% but equal to or less than 0.100% of W, more than 0% but equal toor less than 0.0200% of Ca, more than 0% but equal to or less than0.0200% of Mg, more than 0% but equal to or less than 0.0200% of Zr, andmore than 0% but equal to or less than 0.0200% of REM.
 3. The electricresistance welded steel pipe for a torsion beam according to claim 1,wherein packet grains in the tempered martensite structure have anaverage grain size of 10 μm or less.
 4. The electric resistance weldedsteel pipe for a torsion beam according to claim 1, wherein the wallthickness central portion in the L cross section has a dislocationdensity of 2.0×10¹⁴ m⁻² or less.
 5. The electric resistance welded steelpipe for a torsion beam according to claim 1, which has an outerdiameter of from 50 to 150 mm and a wall thickness of from 2.0 to 4.0mm.
 6. The electric resistance welded steel pipe for a torsion beamaccording to claim 3, which has an outer diameter of from 50 to 150 mmand a wall thickness of from 2.0 to 4.0 mm.
 7. An electric resistancewelded steel pipe for a torsion beam, the steel pipe comprising a basemetal portion and an electric resistance welded portion, wherein achemical composition of the base metal portion comprising, in terms of %by mass: 0.05 to 0.30% of C, 0.03 to 1.20% of Si, 0.30 to 2.50% of Mn, 0to 0.030% of P, 0 to 0.010% of S, 0.010 to 0.200% of Ti, 0.005 to 0.500%of Al, 0.010 to 0.040% of Nb, 0 to 0.006% of N, 0.0005 to 0.0050% of B,0 to 1.000% of Cu, 0 to 1.000% of Ni, 0 to 1.00% of Cr, 0 to 0.50% ofMo, 0 to 0.200% of V, 0 to 0.100% of W, 0 to 0.0200% of Ca, 0 to 0.0200%of Mg, 0 to 0.0200% of Zr, 0 to 0.0200% of REM, and, a balancecomprising Fe and impurities, wherein: V_(c90), defined by the followingFormula (i), is from 2 to 150, a mass ratio of Ti content to N contentis 3.4 or more, a metallographic microstructure of a wall thicknesscentral portion is a tempered martensite structure, and an averageaspect ratio of prior austenite grains in the tempered martensitestructure is 2.0 or less, in an L cross section at a position deviatingby 180° in a circumferential direction of the pipe from the electricresistance welded portion, a metallographic microstructure of an areawithin a distance corresponding to a wall thickness from the electricresistance welded portion in a wall thickness central portion in a Ccross section includes a tempered martensite and at least one of atempered bainite or a ferrite, yield elongation is observed when atensile test in a pipe axis direction is performed, and a tensilestrength in the pipe axis direction is from 750 to 980 MPa:log V_(c90)=2.94−0.75βa  Formula (i)βa=2.7C+0.4Si+Mn+0.45Ni+0.8Cr+2Mo  Formula (ii) wherein, in Formula (i),sa represents a value defined by Formula (ii), and in Formula (ii),element symbols represent % by mass of respective elements.
 8. Theelectric resistance welded steel pipe for a torsion beam according toclaim 7, wherein the chemical composition of the base metal portioncontains, in terms of % by mass, at least one selected from the groupconsisting of: more than 0% but equal to or less than 1.000% of Cu, morethan 0% but equal to or less than 1.000% of Ni, more than 0% but equalto or less than 1.00% of Cr, more than 0% but equal to or less than0.50% of Mo, more than 0% but equal to or less than 0.200% of V, morethan 0% but equal to or less than 0.100% of W, more than 0% but equal toor less than 0.0200% of Ca, more than 0% but equal to or less than0.0200% of Mg, more than 0% but equal to or less than 0.0200% of Zr, andmore than 0% but equal to or less than 0.0200% of REM.
 9. The electricresistance welded steel pipe for a torsion beam according to claim 7,wherein packet grains in the tempered martensite structure have anaverage grain size of 10 μm or less.
 10. The electric resistance weldedsteel pipe for a torsion beam according to claim 7, wherein the wallthickness central portion in the L cross section has a dislocationdensity of 2.0×10¹⁴ m⁻² or less.
 11. The electric resistance weldedsteel pipe for a torsion beam according to claim 7, which has an outerdiameter of from 50 to 150 mm and a wall thickness of from 2.0 to 4.0mm.
 12. The electric resistance welded steel pipe for a torsion beamaccording to claim 9, which has an outer diameter of from 50 to 150 mmand a wall thickness of from 2.0 to 4.0 mm.